Conventionally, techniques for enhancing the strength of field element cores have been proposed. For example, Patent Document 1 cited below discloses a technique in which a slot for insertion of a field magnet is divided into two, and a connecting portion (which is referred to as “a bridge” in Patent Document 1) which connects the outer side and the inner side of the field element core across the slots is provided.
However, the divided slots of Patent Document 1 are rectangular-shaped, and therefore have corners at the ends of the connecting portion. Accordingly, stresses occurring in the connecting portion concentrate in the corners, possibly causing deformation of the connecting portion or further of the field element core.
For example, Patent Document 2 and Patent Document 3 listed below introduce techniques for alleviating the stress concentration by rounding the corners of connecting portions.
Also, other techniques related to the present invention are introduced in Patent Documents 4 and 5.
Patent Document 1: Japanese Utility Model Application Laid-Open No. 7-11859 (1995)
Patent Document 2: Japanese Patent Application Laid-Open No. 2002-281700
Patent Document 3: Japanese Patent Application Laid-Open No. 2004-260888
Patent Document 4: Japanese Patent Application Laid-Open No. 9-294344 (1997)
Patent Document 5: Japanese Patent Application Laid-Open No. 2003-174747